Method for manufacturing an array of thin film actuated mirrors

ABSTRACT

An improved method for manufacturing an array of M×N thin film actuated mirrors comprises the steps of: providing a base having a flat top surface; forming sequentially a separation layer, a first thin film layer, and a thin film electrodisplacive layer on the top surface of the base; heat treating the thin film electrodisplacive layer to allow a phase transition to take place; forming sequentially a second thin film layer, a elastic layer, and a thin film sacrificial layer on top of the thin film electrodisplacive layer; forming an array of M×N empty slots to the sacrificial layer; forming a supporting layer on top of the sacrificial layer; forming an array of M×N conduits, each of the conduits passing through the supporting layer and each of the empty slots to thereby form a multilayer structure; forming an array of M×N transistors on top of the multilayer structure to thereby form a semifinished actuated mirror structure; attaching an active matrix to the semifinished actuated mirror structure; separating the base from the semifinished actuated mirror structure by removing the separation layer to thereby form an actuated mirror structure; patterning the actuated mirror structure into an array of M×N semifinished actuated mirrors; and removing the thin film sacrificial layer to thereby form the array of M×N thin film actuated mirrors.

FIELD OF THE INVENTION

The present invention relates to an optical projection system; and, moreparticularly, to an improved method for manufacturing an array of M×Nthin film actuated mirrors for use in the system.

BACKGROUND OF THE INVENTION

Among the various video display systems available in the art, an opticalprojection system is known to be capable of providing high qualitydisplays in a large scale. In such an optical projection system, lightfrom a lamp is uniformly illuminated onto an array of, e.g., M×N,actuated mirrors, wherein each of the mirrors is coupled with each ofthe actuators. The actuators may be made of an electrodisplacivematerial such as a piezoelectric or an electrostrictive material whichdeforms in response to an electric field applied thereto.

The reflected light beam from each of the mirrors is incident upon anaperture of, e.g., an optical baffle. By applying an electrical signalto each of the actuators, the relative position of each of the mirrorsto the incident light beam is altered, thereby causing a deviation inthe optical path of the reflected beam from each of the mirrors. As theoptical path of each of the reflected beams is varied, the amount oflight reflected from each of the mirrors which passes through theaperture is changed, thereby modulating the intensity of the beam. Themodulated beams through the aperture are transmitted onto a projectionscreen via an appropriate optical device such as a projection lens, tothereby display an image thereon.

In FIGS. 1 and 2A to 2F, there are shown cross sectional viewsillustrating an array 10 of M×N thin film actuated mirrors 11 for use inan optical projection system, wherein M and N are integers, and crosssectional views setting forth a method for manufacturing same, disclosedin a copending commonly owned application, U.S. Ser. No. 08/331,399,entitled "THIN FILM ACTUATED MIRROR ARRAY AND METHOD FOR THE MANUFACTURETHEREOF".

The array 10 of M×N thin film actuated mirrors 11 shown in FIG. 1comprises an array 13 of actuating structures 14, an array 17 of M×Nmirrors 18, an active matrix 12, and an array 15 of M×N supportingmembers 16.

As illustrated in FIG. 2A, the process for manufacturing the array 10 ofM×N thin film actuated mirrors 11 begins with the preparation of theactive matrix 12, having a top and a bottom surfaces 75, 76, comprisinga substrate 59, an array of M×N transistors (not shown), a conductionline pattern (not shown) and an array 60 of M×N connecting terminals 61.

In a subsequent step, there is formed on the top surface 75 of theactive matrix 12 a supporting layer 80, including an array 81 of M×Npedestals 82 corresponding to the array 15 of M×N supporting members 16and a sacrificial area 83, wherein the supporting layer 80 is formed by:depositing a thin film sacrificial layer(not shown) on the entirety ofthe top surface 75 of the active matrix 12; forming an array of M×Nempty slots (not shown), to thereby generate the sacrificial area 83,each of the empty slots being located around each of the M×N connectingterminals 61; and providing a pedestal 82 in each of the empty slots, asshown in FIG. 2B. The thin film sacrificial layer is formed by using asputtering method; the array of empty slots, using an etching method;and the pedestals, using a sputtering or a chemical vapordeposition(CVD) method, followed by an etching method. The sacrificialarea 83 of the supporting layer 80 is then treated for its subsequentremoval using, e.g., an etching method.

A conduit 73 is formed in each of the pedestals 82 by first creating ahole extending from top thereof to top of the corresponding connectingterminals 61 using an etching method, followed by filling therein withan electrically conductive material, as depicted in FIG. 2C.

In the subsequent step, as depicted in FIG. 2D, a first thin filmelectrode layer 84, made of an electrically conductive material, e.g.,Pt, is deposited on the supporting layer 80. Thereafter, a thin filmelectrodisplacive layer 85, made of an electrodisplacive material, e.g.,PZT, is formed on top of the first thin film electrode layer 84. Thestructure is then heat treated at a temperature ranging from 600° C. to800° C. to allow a phase transition to take place in the thin filmelectrodisplacive layer 85. Once the phase transition has taken place, asecond thin film electrode layer 95, made of an electrically conductivematerial, is deposited on top of the thin film electrodisplacive layer85.

Subsequently, a thin film layer 99, made of a light reflecting material,e.g., Al, is provided on top of the second electrode layer 95.

The thin film layers of the electrically conductive, theelectrodisplacive, and the light reflecting materials may be depositedand patterned with the known thin film techniques, such as sputtering,Sol-Gel, evaporation, etching and micromachining, as shown in FIG. 2E.

The sacrificial area 83 of the supporting layer 80 is then removed ordissolved by the application of chemicals, e.g., fluoric acid, tothereby form said array 10 of M×N thin film actuated mirrors 11, asillustrated in FIG. 2F.

There are a number of problems associated with the above describedmethod for manufacturing the array 10 of M×N thin film actuated mirrors11. The first and foremost one is the degradation of the transistors andthe conduction line pattern in the active matrix 12 during the formationof the supporting members 16 and the thin film electrodisplacive layer85, caused by a high temperature required.

SUMMARY OF THE INVENTION

It is, therefore, a primary object of the present invention to provide amethod for manufacturing an array of M×N thin film actuated mirrors foruse in an optical projection system, the method capable of minimizingthe degradation of the transistors and the conduction line pattern inthe active matrix.

In accordance with one aspect of the present invention, there isprovided an improved method for manufacturing an array of M×N thin filmactuated mirrors for use in an optical projection system, said methodcomprising the steps of: (a) providing a base having a top surface; (b)forming a separation layer on the top surface of the base; (c)depositing a first thin film layer, made of an electrically conductiveand light reflecting material, capable of functioning as a mirror aswell as a bias electrode in the thin film actuated mirrors, on top ofthe separation layer; (d) forming a thin film electrodisplacive layer ontop of the first thin film layer; (e) heat treating the thin filmelectrodisplacive layer to allow a phase transition thereof to takeplace; (f) depositing a second thin film layer, made of an electricallyconductive material, capable of functioning as a signal electrode in thethin film actuated mirrors, on top of the thin film electrodisplacivelayer; (g) forming an elastic layer, made of an insulating material, ontop of the second thin film layer; (h) constructing a thin filmsacrificial layer on top of the elastic layer; (i) establishing an arrayof M×N empty slots by removing portions of the sacrificial layer, eachof the empty slots extending from top of the sacrificial layer to top ofthe elastic layer; (j) building a supporting layer, made of apoly-silicon, on top of the sacrificial layer, wherein each of the emptyslots is also filled with the poly-silicon; (k) structuring an array ofM×N conduits, each of the conduits extending from top of the supportinglayer to top of the second thin film layer, each of the conduits passingthrough each of the empty slots to thereby form a multilayer structure;(l) making an array of M×N transistors on top of the multilayerstructure, each of the transistors being electrically connected to eachof the conduits through a conduction line pattern to thereby form asemifinished actuated mirror structure; (m) attaching an active matrixto the semifinished actuated mirror structure; (n) separating the basefrom the semifinished actuated mirror structure by removing theseparation layer to thereby form an actuated mirror structure; (o)patterning the actuated mirror structure into an array of M×Nsemifinished actuated mirrors; and (p) removing the thin filmsacrificial layer in each of the semifinished actuated mirrors tothereby form the array of M×N thin film actuated mirrors.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention willbecome apparent from the following description of preferred embodimentsgiven in conjunction with the accompanying drawings, wherein:

FIG. 1 shows a cross sectional view of an array of M×N thin filmactuated mirrors previously disclosed;

FIGS. 2A to 2F illustrate schematic cross sectional views setting forththe manufacturing steps for the array shown in FIG. 1; and

FIGS. 3A to 3D provide schematic cross sectional views explaining theinventive method for manufacturing an array of M×N thin film actuatedmirrors.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process for manufacturing the array 100 begins with the preparationof a base 110, made of an insulating material, e.g., glass, and having aflat top surface.

In the first step, as shown in FIG. 3A, a separation layer 130, a firstthin film layer 115, a thin film electrodisplacive layer 120, a secondthin film layer 116, an elastic layer 121 and a thin film sacrificiallayer 131 are sequentially formed on the top surface of the base 110.The separation layer 130, made of an water soluble material, e.g., NaCl,and having a thickness of 1000 Å to 3000 Å, is formed on the top surfaceof the base 110 by using a sputtering or a vacuum evaporation method.The first thin film layer 115, made of an electrically conducting andlight reflecting material, e.g., platinum(Pt), capable of functioning asa mirror as well as a bias electrode in the thin film actuated mirrors101, and having a thickness of 500-2000 Å, is formed on top of theseparation layer 130 by using a sputtering or a vacuum evaporationmethod. The thin film electrodisplacive layer 120, made of apiezoelectric material, e.g., lead zirconium titanate(PZT), or anelectrostrictive material, e.g., lead magnesium niobate(PMN), and havinga thickness of 0.7-2 μm, is formed on top of the first thin film layer115 by using a Sol-Gel method or a sputtering method and then is heattreated to allow a phase transition thereof to take place. Since thethin film electrodisplacive layer 120 is sufficiently thin, there is noneed to pole it in case it is made of a piezoelectric material: for itcan be poled with the electrical signal applied during the operation ofthe actuated mirrors 101. The second thin film layer 116, made of anelectrically conducting material, e.g., platinum(Pt) orplatinum/titanium(Pt/Ti), capable of functioning as a signal electrodein the thin film actuated mirrors(101), and having a thickness of 0.7-2μm, is formed on top of the thin film electrodisplacive layer 120 byusing a sputtering or a vacuum evaporation method. The elastic layer121, made of an insulating material, e.g., silicon nitride(Si₃ N₄), andhaving a thickness of 500-2000 Å, is formed on top of the second thinfilm layer 116 by using a Sol-Gel method, a sputtering method or achemical vapor deposition(CVD) method. The thin film sacrificial layer131, made of a metal, e.g., copper(Cu) or nickel(Ni), or aphosphor-silicate glass(PSG) or a polysilicon, and having a thickness of1-2 μm, is formed on top of the elastic layer 121 by using a sputteringmethod if the thin film sacrificial layer 131 is made of a metal, achemical vapor deposition(CVD) method or a spin coating method if thethin film sacrificial layer 131 is made of a PSG, and a CVD method ifthe thin film sacrificial layer 131 is made of a poly-silicon.

In the subsequent step, an array of M×N empty slots(not shown) is formedin the thin film sacrificial layer 131 by using a photolithographymethod, wherein each of the slots extends from top thereof to top of theelastic layer(121). A supporting layer 126, made of an insulatingmaterial, e.g., silicon nitride(Si₃ N₄), is formed on top of the thinfilm sacrificial layer 131 by using a sputtering method or a CVD method,wherein each of the empty slots is also filled with the poly-silicon.Thereafter, an array of M×N conduits 125, made of a metal, e.g.,aluminum(A1), each of the conduits 125 being capable of providing anelectric signal to the second thin film layer 116 in each of theactuated mirrors 101, is formed by: first creating M×N holes(not shown),each of the holes extending from top of the supporting layer 126 to topof the second thin film layer 116 and passing through each of the emptyslots, by using an etching method; and filling therein with the metal byusing a sputtering method, to thereby form a multilayer structure 200,as depicted in FIG. 3B.

Thereafter, an array of M×N transistors 140, made of a semiconductor,e.g., metal oxide semiconductor(MOS), is formed on top of the multilayerstructure 200. Each of the transistors 140 is connected electrically toeach of the conduits 125 through a conduction line pattern 141, made ofa metal, e.g., copper(Cu), to thereby form a semifinished actuatedmirror structure 210, as shown in FIG. 3C. Each of the transistors 140can be directly connected electrically to each of the conduits 125.

In the ensuing step, an active matrix 111, made of a ceramic, isattached to the semifinished actuated mirror structure 210 by using anadhesive 145 made of a metal having a low melting point and a lowcontraction ratio, e.g., indium(In), tin(Sn). The separation layer 130is removed by using a wet etching method, to thereby separate the base110 from the semifinished actuated mirror structure 210 to form anactuated mirror structure(not shown). Thereafter, the actuated mirrorstructure is patterned into an array of M×N semifinished actuatedmirrors(not shown) by using a photolithography method or a lasertrimming method.

The thin film sacrificial layer 131 in each of the semifinished actuatedmirrors is then removed by using an etching method to thereby form thearray 100 of M×N thin film actuated mirrors 101, as shown in FIG. 3D.

In contrast with the method for forming the array of M×N thin filmactuated mirrors disclosed previously, wherein the heat treatment forforcing the phase transition of the electrodisplacive materialconstituting the electrodisplacive layer 85 to occur after the activematrix 12 has been attached, in the inventive method, the heat treatmenttakes place prior to the forming of the transistors 140 and theconduction line pattern 141, thereby preventing the degradation of thetransistors 140 and the conduction line pattern 141 due to the heat.

While the present invention has been described with respect to certainpreferred embodiments only, other modifications and variations may bemade without departing from the scope of the present invention as setforth in the following claims.

What is claimed is:
 1. A method for manufacturing an array of M×N thinfilm actuated mirrors for use in an optical projection system, themethod comprising the steps of:(a) providing a base having a topsurface; (b) forming a separation layer on the top surface of the base;(c) depositing a first thin film layer, made of an electricallyconductive and light reflecting material, capable of functioning as amirror as well as a bias electrode in the thin film actuated mirrors, ontop of the separation layer; (d) forming a thin film electrodisplacivelayer on top of the first thin film layer; (e) heat treating the thinfilm electrodisplacive layer to allow a phase transition thereof to takeplace; (f) depositing a second thin film layer, made of an electricallyconductive material, capable of functioning as a signal electrode in thethin film actuated mirrors, on top of the thin film electrodisplacivelayer; (g) forming an elastic layer, made of an insulating material, ontop of the second thin film layer; (h) forming a thin film sacrificiallayer on top of the elastic layer; (i) forming an array of M×N emptyslots by removing portions of the sacrificial layer, each of the emptyslots extending from top of the sacrificial layer to top of the elasticlayer; (j) forming a supporting layer, made of a poly-silicon, on top ofthe sacrificial layer, wherein each of the empty slots is also filledwith the poly-silicon; (k) forming an array of M×N conduits, each of theconduits extending from top of the supporting layer to top of the secondthin film layer, each of the conduits passing through each of the emptyslots to thereby form a multilayer structure; (l) forming an array ofM×N transistors on top of the multilayer structure, each of thetransistors being electrically connected to each of the conduits througha conduction line pattern to thereby form a semifinished actuated mirrorstructure; (m) attaching an active matrix to the semifinished actuatedmirror structure; (n) separating the base from the semifinished actuatedmirror structure by removing the separation layer to thereby form anactuated mirror structure; (o) patterning the actuated mirror structureinto an array of M×N semifinished actuated mirrors; and (p) removing thethin film sacrificial layer in each of the semifinished actuated mirrorsto thereby form the array of M×N thin film actuated mirrors.
 2. Themethod of claim 1, wherein the separation layer is made of a watersoluble material.
 3. The method of claim 1, wherein the separation layeris formed by using a sputtering or a vacuum evaporation method.
 4. Themethod of claim 1, wherein the separation layer is removed by using awet etching method.
 5. The method of claim 1, wherein the thin filmelectrodisplacive layer is made of a piezoelectric or anelectrostrictive material.
 6. The method of claim 5, wherein the thinfilm electrodisplacive layer is formed by using a Sol-Gel or asputtering method.
 7. The method of claim 1, wherein the first and thesecond thin film layers are formed by using a sputtering or a vacuumevaporation method.
 8. The method of claim 1, wherein the elastic layeris formed by using a Sol-Gel, a sputtering or a CVD method.
 9. Themethod of claim 1, wherein the thin film sacrificial layer is formed byusing a sputtering method if the thin film sacrificial layer is made ofa metal; a chemical vapor deposition method or a spin coating method ifthe thin film sacrificial layer is made of a phosphor-silicate glass; ora chemical vapor deposition method if the thin film sacrificial layer ismade of a poly-silicon.
 10. The method of claim 1, wherein the conduitsare formed by using an etching method, followed by a sputtering method.11. The method of claim 1, wherein the actuated mirror structure ispatterned by using a photolithography or a laser trimming method. 12.The method of claim 1, wherein the thin film sacrificial layer isremoved by using an etching method.
 13. The method of claim 1, whereineach of the transistors is directly connected electrically to each ofthe conduits.